Method of preparing sensitized photographic emulsions



United States Patent 3,276,877 METHOD OF PREPARING SENSITIZED PHOTOGRAPHIC EMULSIONS Henry Clay Yutzy and William F. Smith, Rochester, N.Y.,

assignors to Eastman Kodak Company, Rochester,

N.Y., a corporation of New Jersey No Drawing. Filed July 17, 1964, Ser. No. 383,508

4 Claims. (Cl. 96-108) This application is a continuation-in-part of our applications Serial Nos. 526,321, filed August 3, 1955 and 235,555, filed November 5, 1962, both now abandoned.

This invention relates to photographic emulsions having a particle crystal dislocation value of less than 5 per micron of surface area which emulsions upon exposure form a surface image but little or no internal image, a process for preparing those emulsions and the reversal development of those emulsions.

It is known that the silver halide crystals of the usual photographic emulsions are considerably sensitive to light both internally and on the surface. Photographic emulsions up to the time of this invention have been those having a concentration of dislocations per micron of silver halide surface area considerably more than 5.0 and generally greater than dislocations per micron In certain types of photographic processes it has appeared desirable to employ emulsions in which the surfaces of the silver halide crystals have a sensitivity considerably greater than the internal portions thereof.

One object of our invention is to provide photosensitive silver halide emulsions characterized in that the concentration of dislocations is less than 5.0 per micron of surface area, which emulsions are characterized by low internal sensitivity of the silver halide crystals thereof. Another object of our invention is to provide a process for preparing photosensitive silver halide emulsions having a low concentration of dislocations. A further object of our invention is to provide a method for preparing emulsions which are eminently suited for use in reversal photography both color and black-and-white types. A still further object of our invention is to provide a process involving the reversal development of photographic emulsions having a low particle crystal dislocation value. Other objects of our invention will appear herein.

In accordance with our invention an emulsion is obtained the surface sensitivity of the silver halide crystals of which when developed in a surface developer and measured at a density of 0.1 plus fog is greater by at least 1.75 log E than the internal sensitivity of those crystals as measured by the development thereof, after bleaching, in an internal developer. These emulsions are characterized by having less than 5.0 dislocations per micron of surface area. The determination of particle crystal dislocation values is described in an article entitled Convergent-Beam X-Ray Analysis of Mosaic Structure in Polycrystals by Chester R. Berry which appeared in the Journal of Applied Physics, Volume 27, No. 6, pages 636-639, June 1956. The emulsions in accordance with our invention are characterized by a physical structure in which the concentration of dislocations isless than 5.0 per micron of surface area which value is readily determined by the simple diffraction method as described in that article. As recognized in that article, many schemes have been proposed for determining angular misalignment in crystals and the invention is not limited to the determination described in the Berry. article.

The physical structure of the silver halide crystals of sensitized photographic emulsions governs the relation of their surface sensitivity to their internal sensitivity, the surface sensitivity of those crystals having a dislocation value of less than 5.0 dislocations per micron of Water cc 750 Elon grams 0.31 Sodium sulfite (desiccated) do 39.6 Hydroquinone do 6.0 Sodium carbonate (desiccated) do 18.7 Potassium bromide do 0.86 Citric acid do 0.68 Potassium metabisulfite do 1.5

Cold water to make 1.0 liter.

A typical internal development involves first bleaching the emulsion layer after exposure in aqueous 0.3% potassium ferricyanide solution and drying and then developing for 5 minutes at 65 F. in an internal developer having the following composition:

Water cc 500 Elon grams 2.2 Desiccated sodium sulfite do 96 Hydroquinone do 818 Desiccated sodium carbonate do 48 Potassium bromide do 5 Sodium thiosulfate do 10 Cold water to make 1.0 liter.

Defined in photographic terms the emulsions in accordance with our invention have a surface sensitivity greater by at least 1.75 log E measured at a density of 0.1 plus fog than the internal sensivity. In physical terms the emulsions in accordance with our invention have less than 5.0 dislocations per micron of surface area as determined by a method for determining the substructure or angular misalignment in crystals such as by the simple X-ray diffraction method described by Berry in his article referred to above.

We have found that silver halide crystals of emulsions in accordance with our invention are obtained by processes in which the following conditions are observed:

(1) Ammoniacal silver nitrate and alkali halide are passed simultaneously into a solution of gelatin or a gelatin derivative with stirring.

(2) At least 50% of the alkali halide is added to the solution of gelatin or gelatin derivative while the solution of ammoniacal silver nitrate is being introduced. The best results are obtained when the alkali metal halide is present in but small excess in the solution of gelatin or gelatin derivative. For instance, an excess of no more than about 5 mole percent of the silver present in the reaction mixture.

(3) The emulsion is of the bromoiodide type, the iodide being preferably 0.55 mole percent based on the total silver halide in the emulsion.

(4) The emulsion is chemically sensitized to increase its surface sensitivity. In some cases the chemical sensitizer, e.g. sulfur sensitizer, may be contained in the protein material used in preparing the emulsion.

The best results are obtained with the simultaneous addition of the alkali metal halide and the silver nitrate to the gelatin or gelatin derivative solution within a period of no more than 20 minutes.

The ammoniacal silver nitrate used is prepared by adding to an aqueous solution of silver nitrate an amount of ammonia sufficient to precipitate the silver as silver oxide and to redissolve the same preferably with a slight osmosis or by one of the coagulation techniques, such as described in U.S. Patents 2,618,556, 2,614,928 and 2,489,- 341. In the preparation of the silver halide the protein material can be gelatin or it can be oxidized casein such as described in U.S. Patent 2,691,582 of Lowe and Gates.

It is preferable to add the silver nitrate solution and alkali halide solution simultaneously to the aqueous solution of protein or its derivative over substantially the entire, time of addition. If desired separate solutions of alkali metal bromide and alkali metal iodide may be employed provided they are added substantially simultaneously to the protein solution, the operation being carried out so that most of the time the alkali metal halide is present in the protein solution in small excess.

Afterthe silver halide has been prepared and washed as mentioned above, it is then dispersed in a vehicle which is usually gelatin and is chemically sensitized. The chemical sensitization of silver halide emulsions is Well known in the art and refers to increasing the surface sensitivity of the emulsion by reacting the silver halide with sulfur,

selenium, or tellurium compounds as described in U.S.

Patents 1,574,944, 1,623,499, and 2,410,689 of Sheppard, or by reacting the silver halide with noble metals such as gold, palladium or platinum as described in U.S. Patent 2,448,060 and British Patent No. 636,140 of Trivelli and Smith, or by reacting the silver halide with reducing agents as described in Carroll British Patent 658,592 and Lowe and Jones U.S. Patent 2,518,698, or by various combinations of these as described in Waller et a1. U.S. Patent 2,399,083 and Lowe et al. U.S. Patents 2,743,182 and 2,743,183. Optionally the emulsions may be sensitized with polyethylene glycols such as described in U.S. Patents 2,423,459; 2,441,389; 1,970,578; 2,400,532, 2,240,472or British Patent 443,559. The emulsion may be ripened at an elevated temperature for a time sufiicient to impart the desired properties to the emulsion. After the emulsion has been prepared, it may be coated out onto a support such as a film base of cellulose triacetate, cellulose acetate propionate or the like.

The low crystal dislocation emulsions which are characterized by high surface, low internal sensitivity are primarily employed in photographic films designed for reversal development such as for color films as referred to in U.S. Patents 2,172,262 and 2,252,718 or for mixed grain color films of the type described in U.S. Patents 2,592,243 and 2,673,149. Mixed grain emulsions of this type may be developed as described in U.S. Patent 2,614,925.

Emulsions in accordance with our invention may also be employed in the manufacture of films sensitized to various regions of the spectrum which are adapted for reversal development and in the manufacture of black and White films adapted for use in reversal development processes of the type described in U.S. Patent 1,460,703 of Capstaff.

EXAMPLE I This example describes a method of preparing emulsions having an appreciable number of dislocations per particle which emulsion is characterized by a low ratio of surface to internal sensitivity for comparison with emulsionsin accordance with our invention. The concentration of dislocations per micron was determined in accordance .with the method described in Berrys article. The concentration of dislocations per 4 micron was found to be 14.- In this definition D is the average diameter of the silver halide crystals in microns, b is the length of the Burgers vector and A20 is the width of a spot produced near the wide part of the diffraction ring as described in Berrys article.

Solution A 52 grams of a photographic gelatin, 250 grams of potassium bromide, 3.5 grams of potassium iodide; dissolved in 4500 cc. of distilled water at 50 C.

Solution C 340 grams of silver nitrate were dissolved in distilled water making a total volume of 2750 cc. The temperature was adjusted to 40 C.

Solution C was run into Solution A in approximately 45 minutes with good stirring. The mixture was cooled to 30 C. and the emulsion was acidified by the addition of 6 N sulfuric acid. The emulsion was rendered substantially free of residual water soluble salts by washing by a conventional coagulation technique. After washing, 428 grams photographic gelatin and 'suflicient distilled water were added to make a total weight of 6.5 kilograms. The mixture was stirred at 40 C. at a pH of 6.4 until completely dispersed. The emulsion was then chemically sensitized as described in Example 5 of British Patent 636,234, except the sulfur and gold compounds were added before the second digestion. After digesting to optimum sensitivity, the following coatings were made.

(A) A portion of the emulsion was coated on a cellulose acetate support at a coverage of 570 sq. ft. per mole of silver halide. Separate samples of this coating were exposed on an Eastman 1B Sensitometer; one was processed in the surface developer and the other was bleached with alkali metal ferricyanide and was processed in the internal developer, previously described. The log of the exposure measured at a density of 0.1+fog of the coating processed in the internal developer, was found to be log E=less than 1.75 slower than the strip processed in the surface developer.

(B) Two separate portions of this emulsion were divided out. A red sensitizer was added to one portion and a green sensitizer was added to the second portion. These two portions were blended together and coated as a single layer on a cellulose acetate support as described in the Carroll and Hanson U.S. Patents 2,592,243 and 2,673,149 resulting in a mixed grain color film.

EXAMPLE II This example illustrates the preparation of an emulsion in accordance with our invention which emulsion is characterized by a high ratio of surface to internal sensitivity and the concentration of dislocations per. micron of surface area was determined to be 4. 8 by the simple X-ray diifraction method described in Berrys article. The average grain size of the silver halide grains obtained was less than 1 micron diameter.

a Solution A 52 grams of a photographic gelatin were dissolved in 3500 cc. of distilled water at 40 C.

Solution B 250 grams of potassium bromide, and 3.5 grams of potassium iodide were dissolved in suflicient distilled water to make a total volume of 1020 cc. at 40 C.

Solution C 340 grams of silver nitrate were dissolved in 1600 grams of distilled water; 300 cc. of 28% ammonium hydroxide were added and the total volume was adjusted to 2850 cc. with distilled water and the temperature was raised to 40 C.

Solutions B and C were run into Solution A in approximately 1'1 minutes with good stirring. The running of Solution B was started 15 seconds before, that of Solution C. After the addition of B and C the mixture was cooled to 30 C. and the emulsion was acidified by the addition of 6 N sulfuric acid. The emulsion was rendered substantially free of residual water soluble salts by Washing by a conventional coagulation technique and after washing, 428 grams of photographic gelatin and sufficient distilled water were added, to make a total weight of 6.5 kilograms. The mixture was stirred at 40 C. and at a pH of 6.4 until completed dispersed. The emulsion was chemically sensitized as in the previous example. The emulsion was digested to optimum sensitivity and the following coatings were made:

(A) A portion of the above emulsion was coated on a cellulose acetate support at a coverage of approximately 570 sq. ft. per mole of silver halide. Samples of this coating were exposed and processed in internal and surface developers as described in Example I and this emulsion was found to have a surface sensitivity log E=2.0 greater than the internal sensitivity when measured at a density of 0.1+fog in the manner previously described.

(B) Separate portions of this emulsion were red and green sensitized respectively and a mixed grain color coating was made as described in Example I in part B.

EXAMPLE III The pressure sensitivity of the coatings described above were determined by placing a strip of each coating on a rigid metal surface and running a narrow weighted wheel along the center of the coating. The diameter of the wheel was 1 5 in. and the width of the wheel was in. A weighted arm attached to a fulcrum rested on the wheel. The weight at the end of the arm was 2035 grams, and the distance along the arm from the wheel to the fulcrum was 1 inch. The distance from the fulcrum to the weight was 23 /2 in. After the coatings were pressured by the wheel in the manner described, the strips were exposed on a sensitometer. The exposure was such that part of the sensitomet-ric strip was in the pressured region and part of the strip was in the untreated region, so that the sensitometric characteristics of both regions could be determined. This was done with the following results:

(a) The single layer film coatings described in Examples I-A and II-A were exposed and developed to a cyan reversal image as follows:

Preconditioning 24 hrs. at 50% R.H. Pressure wheel run down center of strip. Sensitomet-ric exposure.

'MQ developer, 3 min.

Wash, 2 min.

(6) White light re-exposure. seconds through base. (7) Cyan color developer, 5 min. (8) Wash5 min.

(9) Bleach-3 min. (10) Fix2 min. (11) Wash-10 min.

The pressure characteristics of these coatings are tabulated below. The changes in maximum density, the change in high-light density and the log E changes measured at a 0.2 density above fog of the untreated film are as follows:

(b) These two coatings were also processed in the following black and white reversal system with the indicated results:

6 Precondition 24 hrs. at 50% R.H. Pressure wheel run down center of strip. Sensitometric exposure. MQ developer-3 min. Wash-2 min. KM O bleach4 min. Clearing bath3 min. (8) Wash3 min. (9) White light re-exposure-l0 seconds. (10) Redeveloper5 min. (11) Acid fix-3 min. (12) Wash-10 min.

Change in Change in Change in Coating Maximum High-Light Log E meas- Density Density ured at 0.2

Above Fog Example IA 02 04 16 Example IIA 09 02 02 (c) The mixed grain coatings described above were processed in the following mixed grain reversal process with the indicated results:

(1) Preconditioning 24 hrs. at 50% R.H.

(2) Pressure wheel run down center of strip (3) Sensitometric exposure (4) MQ developer-3 min.

(5 Wash-2 min. 7

(6) Red re-exposure through red filter-5 seconds (7) Cyan color developer-5 min. 50 seconds (8) Auxiliary developer-50 seconds (9) Washl min.

(10) Green re-exposure through yellow filter-30 seconds (11) Magenta color developer-5 min.

(12) Wash5 min.

(13) Bleach--3 min.

(14) Fix2 min.

(15) Wash-10 min.

Change in Change in Change in Coating Maximum High-Light Log E meas- Density Density ured at 0.2

Above Fog Example 18 (Cyan) 18 06 20 Example IIB (Cyan) 03 0 02 Example 113 (Magenta) 14 06 12 Example IIB (Magenta) 03 0 03 It can be seen quite clearly from the above results that emulsions having a high ratio of surface to internal sensitivity are much less sensitive to pressure than emulsions having a low ratio of surface to internal sensitivity. The solutions used for processing the coatings in Example III are as follows:

7 6-nitrobenzamidazol nitrate .02 KI gm .002 Hydroxylamine hydrochloride gm 1.5 2,4-dichloro--(p toluene sulfonyl amino-lnaphthol) gm .5 NaOH gm .5 pH 10.0

Auxiliary developer:

Sodium hexa metaphosphate gm 1.0 N'd2sO3 gm Elon gm 3.0 Na CO gm 25.0 NaBr gm 4.3 NaOH gm. 1.55 6-nitrobenzamidazol nitrate gm .1 pH 11.5

Magenta color developer:

Sodium hexa metaphosphate gm .5 Na2s03 IH 4-amino, N-ethyl, N-(beta-methane sulfonamido ethyl) m toluidine sesquisulfate monohydrate gm 3.5 Nazcoa gm KBr gm .22 N21 SO gm 50.0 6-'nitrobenzamidazol nitrate gm .01 KI gm .002 1-(2' quinolyl) 3 benzamido 5 pyrazolone gm 1.0 NaOH gm .8 pH 10.7

Bleach:

Sodium hexa metaphosphate gm 1.0 K FE(CN) gm 80.0 NaBr gm 34.4 Fix:

Sodium hexa metaphosphate gm 10.0 Na SO gm 10.0 Sodium thiosulfate gm 320.0 KMnO bleach:

KMnO gm 2.5 H2804 CC AgNO gm 0.3 Clearing bath:

Sodium hexa metaphosphate gm 0.6 NaHSO gm 15.0 Redeveloper:

Sodium hexa metaphosphate gm 0.6 Na SO gm 70.0 Elon gm 1.45 Hydroquinone gm 5.6 Na CO gm 39.7 KBr gm 1.1 Acid fix:

Sodium thiosulfate gm 240.0 Sodium sulfite, desiccated gm 15.0 Acetic acid, 28% cc 48.0 Boric acid, crystals gm 7.5 Potassium alum gm 15.0

EXAMPLE IV This example is another example of a method of preparing photographic emulsions having less than 5.0 dislocations per micron of surface area. The concentration of dislocations per micron of surface area was determined by the X-raydiifraction method described in Berrys article and was found to be 4.8.

Solution A 525 grams of a photographic gelatin were dissolved in 8750 cc. of distilled water at 45 C.

. to cool to 28 C. and was then Solution B 625 grams of potassium bromide and 8.75 grams of potassium iodide were dissolved in distilled water to form a a total volume of 2440 cc. at 40 C.

Solution C 850 grams of silver nitrate were dissolved in 4000 cc. of distilled water. There was then added 750 cc. of 28% ammonium hydroxide by which a precipitate was formed and was redissolved. The volume of the solution was brought to 6750 cc. by adding distilled water at a temperature of 40 C.

Solutions B and C were run simultaneously into Solution A over a period of approximately 11 minutes, the addition of Solution B having been started 15 seconds before that of Solution C. The mass was stirred mechanically during the mixing. After the formation .of the silver halide dispersion the mass was cooled to 28 C. and the emulsion was acidified by the addition of 6 N sulfuric acid. The emulsion was rendered substantially free of residual water soluble salts by washing by a conventional coagulation technique. After washing 875 grams of photographic gelatin and suflicient distilled water were added to make a total weight of 19.5 kilograms. The emulsion was dispersed at 40 C. and at a pH. of 6.4 and 285 cc.

EXAMPLE V This example is offered as a method of preparing photographic emulsions in accordance with our invention. The concentration of dislocations per micron of surface area of the emulsion grains thus prepared was found to be 4.0 when the emulsion thus prepared was examined by the X-ray. diffraction method described in Berrys article. These grains had an average size of less than 1 micron diameter.

Solution A 52 grams of a photographic gelatin were dissolved in 3500 cc. of distilled water at 45 C.

Solution B 25 grams of potassium bromide and 3.5 grams of potassium iodide were dissilved in cc. of distilled water at 40 C.

Solution C 340 grams of silver nitrate were dissolved in 1600 cc. of distilled water and there was added thereto 300 cc. of 28% ammonium hydroxide which .additioncaused the formation of a precipitate and redissolving thereof. The total volume was adjusted to 2700 cc.- by adding distilled water at 40 C.

Solution E 225 grams of potassium bromide were cc. of distilled water at 40 C.

Solutions B and C were run into Solution A.with good mechanical stirring, the introduction of Solution B having been started approximately 10 seconds before Solution C. Solution B was run in completely over 1 minute and then Solution E was run into Solution A, this taking approximately 9 minutes. The running of Solution C into A was over a period of 11 minutes so that the halides and the silver solution were both run into Solution Aover substantially the entire period- After the solutions had all been mixed together. the mass thus formed was allowed acidified by the addition of6 N sulfuric acid. The emulsion was substantially freed of residual water tional coagulation technique.

dissolved in 790 1 After washing, 428 grams of photographic gelatin and suflicient distilled water to bring the total weight to 7.8 kilograms were added and the emulsion was dispersed at 40 C. at a pH of 6.4. 114 cc. of a 10% solution of salicylamide were added and the emulsion was chemically sensitized as described in EX- ample I. The mass was then cooled to 40 C. A coating aid was added and the emulsion was applied as a layer on cellulose acetate film support at the rate of 570 sq. ft. per mol of silver.

EXAMPLE VI An emulsion was prepared by the method described in Example II except that Solutions B and C were run into Solution A in approximately 2 minutes instead of 11 minutes. The silver halide grains obtained had an average size less than 1 micron diameter. An emulsion was obtained which, upon testing by the X-ray diffraction method of Berry, averaged less than 3.0 dislocations per micron of silver halide surface area computed from the formula:

Concentration of dislocations per micron in which D is grain diameter in microns b is the length of the Burgers vector and A20 is the diffraction spot width in radians which is independent of the dimensions of the X-ray source.

Three strips of each of the photographic films as prepared in Examples I and II were given a positive exposure for of a second in an Ib sensitometer and one sample of each after potassium ferricyanide bleach treatment was processed in an internal developer, a second of each in a surface developer, of the formulas given above, the development in the internal developer being for 5 minutes and that in the surface developer for 6 minutes. The third strips of each were given a cyan reversal development. The surface and internal sensitivities of the emulsion produced in accordance with the process of Example I were determined as described above. These sensitivities as expressed in log E values differed by somewhat less than 1.75. This difference as determined in the case of the emulsion produced in accordance with the process of Example II was well above 1.75 in terms of log E values.

The third strips of each of the films as prepared in Examples I and II after exposure was bathed in a prehardening bath for 3 minutes having the following composition:

Sodium hexametaphosphate grams 0.5 Sodium bisulfite do Formaldehyde 40% solution cc 27 Sodium carbonate grams 10 Sodium sulfate do 100 Potassium bromide do 2.5

Water to make 1 liter.

The strips were washed and then developed in a developer of the following composition for 8 minutes:

l-phenyl-5-mercaptotetrazole 0.01 Water to make 1 liter.

The film strips were washed and flashed to white light. The film strips were then developed in a cyan developer having the following composition:

Grams Sodium hexametaphosphate 0.5 Sodium sulfite (dessicated) 5.0 Isopropyl naphthalene sodium sulfonate 0.2 Z-amino-5-diethylamino-toluene hydrochloride 1.0

10 Sodium carbonate monohydrate 15.0 Potassium bromide 0.25 Sodium hydroxide 0.5 2,4 dichloro 5 (p toluene sulfonyl amino l naphthol) 1.4 Sodium monohydroquinone sulfonate 0.6

Water to make 1 liter.

The film strips were washed and then bleached in a bath having the following composition:

Grams Sodium hexametaphosphate 1 Potassium ferricyanide Potassium bromide 40 Water to make 1 liter.

The film strips were then thoroughly washed and dried. There was obtained a direct positive image in color. A determination of the maximum density showed the emulsion prepared in accordance with Example I to have a density of 3.10 and that from Example II greater than 4.6.

In the determination of the particle crystal dislocation by the formula Number of particle crystal dislocations the diffraction spot value, A20, in radians (also called angular misalignment) is determined conveniently by the X-ray diffraction method described in the Berry article in Journal of Applied Physics referred to above.

The particle diameter, D, is conveniently determined by observing the projected diameter in microns of the grains from a photomicrograph thereof and then dividing by the degree of magnification which was used. However, other methods of determining the particle diameters of the grains are available. For instance, this value may be determined by a method involving counting of the number of spots as described in the book X-rays in Practice by Wayne T. Sproull, pages 440446, McGraw- Hill, 1946.

The value b, length of the Burgers vector, is constant with the particular crystal type being considered. With the crystal structure of silver halide grains the value b is 4.0 Angstrom units. Thus the formula for the determinations here is N (per micron) The invention has been described in considerable detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove, and as defined in the appended claims.

We claim:

1. A method of preparing photographic emulsions, the silver halide crystals of which have high surface and low internal sensitivity, which comprises mixing with an aqueous solution of a protein peptizer for silver halide, alkali metal halide in aqueous solution, which halide essentially consists of bromide and iodide of which the latter constitutes 0.5-5 mole percent and ammoniacal silver nitrate, at least 50% of the alkali metal halide which reacts with the ammoniacal silver nitrate being added simultaneously therewith to the protein peptizer solution over a period of not more than 20 minutes, removing residual water soluble salts from the silver halide crystals which are formed thereby and digesting the mass to a selected photosensitivity.

2. A method of preparing photographic emulsions, the silver halide crystals of which have high surface and low internal sensitivity, which comprises mixing with an aqueous solution of a protein peptizer for silver halide, alkali metal halide in aqueous solution, which halide essentially consists of bromide and iodide of which the latter constitutes 0.5- mole percent, and ammoniacal silver nitrate, at least 50% of the alkali metal halide which will react with the silver nitrate being added simultaneously therewith to the protein peptizer solution over a period of not more than 20 minutes, removing residual water soluble salts from the silver halide crystals formed thereby and digesting those crystals with a sulfur sensitizer to a selected photosensitivity.

3. A method of preparing photographic emulsions, the silver halide crystals of which have high surface and low internal sensitivity, which comprises mixing with an aqueous solution of a protein peptizer for silver halide, alkali metal halide in aqueous solution, which halide essentially consists of bromide and iodide of which the latter constitutes 0.5 5 mole percent, and ammoniacal silver nitrate, at least 50% of the alkali metal halide which reacts with the ammoniacal silver nitrate being added simultaneously therewith to the protein peptizer solution over a period of not more than 20 minutes, removing residual water soluble salts from the silver halide crystals formed thereby and digesting those crystals with a sulfur sensitizer and a gold sensitizer to a selected photosensitivity.

4. t A method of preparing photographic emulsions, the silver halide crystals of which have high surface and low internal sensitivity, which comprises mixing with an aqueous solution of gelatin, alkali metal halide in aqueous solution, which halide consists of bromide and iodide of which the latter constitutes 0.55 mole percent, and ammoniacal silver nitrate, at least of the alkali metal halide which reacts with the ammoniaca-l silver nitrate being added simultaneously therewith to the aqueous gelatin solution over a period of not more than 20 minutes, removing residual water, soluble salts from the silver halide crystals formed and digesting the silver halide crystals with a sulfur sensitizer and a gold sensitizer to a selected photosensitivity.

References Cited by the Examiner UNITED STATES PATENTS 12/1948 Knott et a1. 9622 12/1949 Hanson 9619 OTHER REFERENCES NORMAN G. TORCHIN, Primary Examiner.

J. RAUBITSCHEK, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,276,877 October 4, 1966 Henry Clay Yutzy et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 41, for "2,423,459" read 2,423,549

column 6, lines 57 and 68, column 7, lines 9, 18, 33, 38, 42: 47, 50 and 58, insert as a heading over the right-hand column,

each occurrence, 1 Liter column 7, line 1, before ".02" insert gm column 8, line 49, for "dissilve'd" read dissolved Signed and sealed this 5th day of September 1967.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A METHIOD OF PREPARING PHOTOGRAPHIC EMULSIONS, THE SILVER HALIDE CRYSTALS OF WHICH HAVE HIGH SURFACE AND LOW INTERNAL SENSITIVITY, WHICH COMPRISES MIXING WITH AN AQUEOUS SOLUTION OF A PROTEIN PEPTIZER FOR SILVER HALIDE, ALKALI METAL HALIDE IN AQUEOUS SOLUTION, WHICH HALIDE ESSENTIALLY CONSISTS OF BROMIDE AND IODIDE OF WHICH THE LATTER CONSTITUTES 0.5-5 MOLE PERCENT AND AMMONIACAL SILVER NITRATE, AT LEAST 50% OF THE ALKALI METAL HALIDE WHICH REACTS WITH THE AMMONIACAL SILVER NITRATE BEING ADDED SIMULTANEOUSLY THEREWITH TO THE PROTEIN PEPTIZER SOLUTION OVER A PERIOD OF NOT MORE THAN 20 MINUTES, REMOVING RESIDUAL WATER SOLUBLE SALTS FROM THE SILVER HALIDE CRYSTALS WHICH ARE FORMED THEREBY AND DIGESTING THE MASS OF A SELECTED PHOTOSENSITIVITY. 